1. Field of the Invention
The present invention relates to time division multiple access (TDMA) satellite communication systems and methods, and more particularly, to all digital TDMA satellite communication systems and methods having dynamic channel allocation.
2. Description of the Related Art
A generalized satellite service was initiated by the assignee to its customers in 1974. The general service was a basic voice-oriented satellite communication network, serving private business exchanges (PBXs) and dedicated trunk linkes. As shown in FIG. 1, it was comprised of customer equipment, local loops, central offices, microwave interconnect, a satellite, and associated earth stations.
Soon after the introduction of the generalized service, a number of specialized networks were also introduced by the assignee. These satellite networks were dedicated and tailored to individual customer needs. Two examples are shown in FIGS. 2 and 3. These dedicated networks include earth stations located on customer premises and baseband multiplex equipment located adjacent to the customer equipment.
As a result of this experience, the assignee realized the need and turned its attention to the development of a special class of dedicated satellite communications service called SDX (SDX is a registered trademark of American Satellite Company for customer premised service). This innovation was characterized by a 5-meter earth station providing a 56-kilobit per second (Kbps) point-to-point duplex service, as shown in FIG. 4.
The SDX satellite communication service was followed by a frequency division multiplex/frequency division multiple access (FDM/FDMA) satellite communications system. The FDM/FDMA system is shown in FIGS. 5 and 6. It consists of analog multiplex equipment performing several stage multiplexing. The composite baseband is sent to earth station via an analog microwave link, where it is frequency modulated (FM) for satellite transmission. Various earth stations in this network share the satellite in a FDMA mode. FIG. 6 shows how this service is integrated into the associated earth station.
A time division multiplex/time division multiple access (TDM/TDMA) communications system shown in FIG. 7 is arranged quite differently. Such a TDM/TDMA system has been found to exhibit greater versatility as compared to the FMD/FDMA communications system discussed above. In the TDM/TDMA communication system, voice circuits are digitized by using unique channel bank equipment into a standard T1 stream. The T1 strems are multiplexed and transported to the earth station via the digital microwave link, where they are converted back to the original T1 lines. The TDMA equipment provides 4 phase modulation (for T1 streams) and network timing such that each terminal can access the full transponder (satellite) at a periodic rate and without interference to other terminals.
A block diagram of a conventional TDMA satellite communications system is shown in FIG. 8. The TDMA equipment operates at a bit rate of 64 Mbps with a frame period of 750 microseconds. The frame format is shown in FIG. 9. At this bit rate, each 36 megahertz (MHz) satellite transponder supports up to 39 T1 data streams. Higher network throughput (up to 250 Mbps) is obtained by utilizing a multiple transponder hopping (receive and/or transmit) feature. Rate 7/8 forward error correcting code (FEC) is applied selectively on a per T1 channel basis to achieve a BER (bit error rate) improvement of better than 3 orders of magnitude. The system utilizes two reference sync bursts to obtain greater system reliability. A 5.33 Kbps alarm channel is also included for centralizing at the operational control center all alarms from the central offices, microwave repeaters, and earth stations. In this regard, reference is made to the article by E. J. Habib and S. Mittal entitled "A New Integrated Service For American Satellite Network," Publication No. AIAA-81-0617-CP, AIAA 9th Communications Satellite Systems Conference, Mar. 7-11, 1982.
The TDM/TDMA system described above, however, does not meet needs of certain users for whom a specialized service is more applicable. Such specialized satellite communications networks can have one of several forms, as shown in FIG. 10: simplex (one way) transmission; point-to-point duplex; broadcast; and, multiway. The fundamental characteristic of these specialized networks is that they are dedicated to the user with on-premise earth stations and utilize fixed single-channel, per-carrier type of transmission. Such specialized systems, however, do not meet the current requirements of the marketplace because requirements have grown to the point where a dynamic reassignment of channel resources per network is complicated and has become a large economic factor.